How to merge two hand perspectives?
I would just go for some retopo scheme.  If you overlap and align them both you can then build up a new mesh using conventional methods for generating clean meshes from ones that are not so clean.  

The problem you are facing is very similar to the issues facing Dynatopo sculpts so the solutions you have are pretty much the same as well.

Once they are aligned properly with each other, you can select them both and merge them into one object, Ctrl-J.  Next you can use the CG Cookie Contours tool to rough out most of the new mesh that will be based on these two as one base.

There are also other methods that you can employ without using Contours.  You can just start with a point in edit mode and extrude it out along the surface of the scanned models and by using the Magnet tool you can keep the points moving along the surface of the base mesh.  

Also, if you model a low-poly, rough hand shaped model, you can use the Shrink Wrap modifier to project the new mesh onto the scanned models.